StratCat's on the right heading. Idle-up is curved to try and keep the rotor speed constant, and it also changes the pitch curve, usually so you have equal positive and negative pitch available. It's normally used for aerobatics, but higher-end radios have more than one idle-up position. I use mine with normal for hovering and general cruising around, idle-1 for faster, more fun stuff, and idle-2 for full-up aerobatics.